1. Field of the Invention
The present invention relates to a liquid tank for holding a liquid used for recording. More particularly, the invention relates to a liquid tank for ink-jet recording which can smoothly and sufficiently supply an ink-jet head with ink when it is mounted on the ink-jet head.
2. Description of the Related Art
Conventionally, a liquid tank for accommodating a liquid used for recording (hereinafter termed an "ink tank" or an "ink cartridge") is integrated with an ink-jet head. When ink within the cartridge becomes incapable of being discharged, the ink tank is, in most cases, disposed together with the head. The amount of ink remaining within the cartridge in this stage depends on the ink holding capability of a sponge, serving as a negative pressure producing material, accommodated in substantially the entire space within the cartridge, and is relatively large even if it is intended to improve the cartridge.
An ink tank or an ink receptacle of this kind is disclosed in Japanese Patent Laid-Open Application (Kokai) No. 63-87242 (1988). This ink receptacle incorporates a foamed material, and constitutes a cartridge integrated with an ink-jet head including a plurality of ink discharging orifices. In this ink receptacle, by storing ink in a porous medium, such as foamed polyurethane, serving as the foamed material, a negative pressure is generated due to the capillary force of foams, and the ink is held to prevent leakage of the ink from the ink receptacle.
However, since it is necessary to load the foamed material in substantially the entire space of the unique ink receptacle, the amount of filled ink is restricted, and the amount of ink remaining in the foamed material without being used is relatively large. Accordingly, the use of the foamed naterial becomes inefficient due to the amount of ink retained by the foamed material. There exist also the problems that it is difficult to detect the amount of remaining ink, and that the negative pressure gradually changes while the ink is being consumed, so that it is difficult to maintain a substantially constant negative pressure for a long period.
In contrast to this configuration, an ink cartridge which holds substantially only ink is disclosed in Japanese Patent Laid-Open Application (Kokai) No. 2-522 (1990). In this ink cartridge which is integrated with an ink-jet head, a small porous member is disposed between a primary ink tank for holding only a large amount of ink which is provided at an upper portion, and the ink-jet head provided at a lower portion. It is claimed that this ink cartridge can improve the efficiency of use of ink because the porous member is disposed only in an ink channel instead of being incorporated within the ink tank. It is also claimed that, by providing a secondary ink tank, serving as a space capable of holding ink, at a side of the porous member, ink drawn from the primary ink tank due to the expansion of the air within the primary tank caused by temperature rise (a decrease in the pressure) is stored in the secondary ink tank, so that the negative pressure for the print head during printing can be maintained substantially constant.
However, in this ink cartridge, since excessive ink is impregnated in the porous member from the primary ink tank for holding only a large amount of ink which is provided at the upper portion, a negative pressure is hardly generated in the porous member. Hence, there is the possibility that ink leaks from an orifice of the ink-jet head by a slight jolt. Hence, this ink cartridge is not suitable for practical use. If an exchangeable ink cartridge which is mounted on an ink print head is adopted in this configuration, ink leaks because of the above-described state of the porous member. Hence, this cartridge is not practially for use.
An ink cartridge, in which ink is sealed within a bag, and a spring for maintaining the negative pressure of the bag constant is provided, is also known. However, this configuration increases the production cost, and it is difficult to achieve mass production of such ink cartridges while maintaining the performance of the spring.
As described above, none of conventional (non-contact-printing) ink cartridges for ink-jet printing are inexpensive and have a rational technical level.
The assignee of the present application has proposed, for example, in U.S. Pat. Nos. 5,509,140 and 5,619,238, ink receptacles suitable for the technical field of ink-jet printing which satisfy the conditions of excellent supply of ink corresponding to the amount of ink discharged from a head during printing, a high efficiency of use of ink, and occurrence of no problems, such as leakage of ink from a discharging port, and the like, while printing is not performed. Such an ink receptacle includes a first chamber incoporating a negative pressure producing material and including an air communication port for obtaining communication with atmospheric air, and a second chamber for directly accommodating ink to be supplied to the first chamber in a substantially closed state although communicating with the first chamber only via a communication port. The communicating port is provided at a part of a partition wall for separating the first chamber from the second chamber.
In this ink receptacle, ink is consumed when the ink is supplied to the ink-jet head side via an ink outlet provided in the first chamber. At the moment when a part of the liquid surface of the ink in the first chamber reaches the upper portion of the communication port, the inside of the second chamber which has been in a substantially closed state starts to communicate with atmospheric air to supply an air bubble into the second chamber. At the same time, the ink in the second chamber is supplied to the first chamber via the communication port. Mutual supply of the gas (an air bubble) and the liquid (ink) at the communication port will be hereinafter termed gas-liquid exchange. In the receptacle having this configuration, gas-liquid exchange is performed, so that the ink within the second chamber is supplied and consumed.
Since this configuration has a tank structure which can maintain the negative pressure substantially constant (at least while the ink within the second chamber is being consumed) during most of the time from the start of use to the end of use of the ink-jet cartridge, it is possible to provide a cartridge for ink-jet recording which can be used even for high-speed printing.
In the ink cartridge having the above-described configuration, the size of the air bubble generated during gas-liquid exchange while the ink is being consumed greatly increases depending on the shape of the opening of the communication port, and the surface tension and the viscosity of the air bubble which depend on the type, the components and the like of the accommodated ink, and the grown air bubble may remain in the communication port. In such a case, there is the possibility that gas-liquid exchange via the communication port is hindered from stopping the supply of ink from the first chamber to the second chamber.
However, since the shape and the size of the opening of the communication port are limited by various factors, such as the external shape of the cartridge, and the like, there is little room for changing the shape and the size. Furthermore, characteristics, such as the surface tension of ink, and the like, are determined by the use of the cartridge, and the like.